Assuming you are referring to a cow that is 20 years old, then the answer to this question would be around 18 calves, if she's never had twins in her life. This is because it takes two years for a female, from birth to her first parturition, to grow old enough to produce a calf. After she has had her first calf she is able to produce an average of one calf per year. The number may be higher if she has had twins in her lifetime.